1. Field of the Invention
This invention relates to the field of wireless communication systems. In particular, the invention relates to systems and methods for linearizing power amplifiers using digital predistortion.
2. Description of the Prior Art and Related Background Information
In modern wireless communication systems, complex signals are created in a digital format, converted into a baseband or analog signal, modulated onto an RF carrier, and amplified prior to signal transmission. The complex signals involved include both amplitude and phase modulation components. When signals vary in amplitude, the data conversion, modulation, and RF amplification circuits must be linear to maintain spectrum emissions within government-regulated compliance. The linearity of practical circuits however is limited particularly if these circuits are driven near operating limits. To maintain reasonable system cost and operating efficiency, operating components near operating limits is desired. One method of providing adequate system linearity and efficiency while operating near component limits is to provide signal predistortion prior to signal conversion, modulation, and amplification. Predistortion modifies the source signal to remove the non-linear effects of signal conversion, modulation, and amplification.
In most conventional predistortion circuits, instantaneous predistortion corrections are multiplied onto the source waveform. In such an approach, instantaneous predistortion corrections are determined by calculating the inverse instantaneous gain from the predistorter input to the amplifier output. This inverse gain is calculated by taking the ratio of the predistorter input divided by the amplifier output. The instantaneous predistortion corrections calculated in this manner are then multiplied onto the desired signal. Errors in the measurement and calculation of predistortion corrections are therefore multiplied onto the desired signal. The impact of signal measurement and calculation errors on the input signal can be quite significant and can actually degrade the signal quality, rather than improving it, in some cases.
In particular, the errors introduced in the above mentioned approach can be significant when the source signal or the output signal become very small or cross through zero. If the output signal crosses through zero or becomes very small, and the input signal does not, predistortion corrections will be based on infinite or very large gain calculations. If the input signal crosses through zero or becomes very small, and the output signal does not, predistortion corrections will be based on very small or zero gain calculations. Neither of these large or small gain calculations is real. Instantaneous predistortion calculations based on these values will be incorrect.
An additional but similar problem with the conventional approach is that all practical amplifiers have memory. This means that, after compensating for the delay of the amplifier, the output signal almost never crosses through zero at the same time as the input signal. Even if no errors existed in input and output signal measurements, amplifier memory effects may cause very large and very small gain calculations to be made when either the input or output signals become small. Also, generally speaking any calculational technique used in a digital predistortion system has inherent limitations in accuracy due to the finite signal quantization and the finite calculational capabilities of the calculation circuitry or processor used in the specific implementation. Ideally, these errors should affect the signal as little as possible without taking away the ability to provide the needed degree of predistortion.
Accordingly, a need presently exists for a digital predistortion system and method which avoids the above noted disadvantages of the prior art.